Hec-ras weir coefficients

[BRIS]

I have received results of hec-ras river modelling showing impacts of adding 1.0m high weirs designed to improve aeration in a river. I am surprised to find that the provision of a 1.0m high weir with a length equal to the full width of the river has created a 1.0m rise in water level.

The weirs are broad crested and should have a Cd of about 1.7 (I understand the weir equation used is Q = Cd.L.H^3/2 in metric units). However the operator tells me that he cannot input a coefficient of greater than 1.4 otherwise the run crashes.

Are we missing something ? Why can we not have a weir coefficient greater than 1.4 ?

 

[SMIAH]

The operator is missing something.

 

[beej67]

If your flow is subcritical, then the weir might even lower the water level due to choking effects.

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